BGP Convergence

Jennifer Rexford

Outline

• Border Gateway Protocol (BGP)– Prefix-based routing at the AS level– Policy-based path-vector protocol– Incremental protocol with update messages

• BGP convergence– Causes of BGP routing changes– Path exploration during convergence– Minimum route advertisement timer

• BGP does not necessarily converge– Bistable routing configurations– Protocol oscillation

Interdomain Routing

• AS-level topology– Destinations are IP prefixes (e.g., 12.0.0.0/8)– Nodes are Autonomous Systems (ASes)– Links are connections and business

relationships

1

2

34

5

67

Client Web server

Challenges for Interdomain Routing

• Scale– Prefixes: 150,000-200,000, and growing– ASes: 20,000 visible ones, and growing– AS paths and routers: at least in the

millions…• Privacy

– ASes don’t want to divulge internal topologies– … or their business relationships with

neighbors• Policy

– No Internet-wide notion of a link cost metric– Need control over where you send traffic– … and who can send traffic through you

Shortest-Path Routing is Restrictive

• All traffic must travel on shortest paths• All nodes need common notion of link

costs• Incompatible with commercial

relationships

Regional ISP1

Regional ISP2

Regional ISP3

Cust1Cust3 Cust2

National ISP1

National ISP2

YES

NO

Link-State Routing is Problematic

• Topology information is flooded – High bandwidth and storage overhead– Forces nodes to divulge sensitive

information

• Entire path computed locally per node– High processing overhead in a large network

• Minimizes some notion of total distance– Works only if policy is shared and uniform

• Typically used only inside an AS– E.g., OSPF and IS-IS

Distance Vector is on the Right Track

• Advantages– Hides details of the network topology– Nodes determine only “next hop” toward

the dest

• Disadvantages– Minimizes some notion of total distance,

which is difficult in an interdomain setting– Slow convergence due to the counting-to-

infinity problem (“bad news travels slowly”)

• Idea: extend the notion of a distance vector

Path-Vector Routing

• Extension of distance-vector routing– Support flexible routing policies– Avoid count-to-infinity problem

• Key idea: advertise the entire path– Distance vector: send distance metric per dest

d– Path vector: send the entire path for each dest

d

32 1

d

“d: path (2,1)” “d: path (1)”

data traffic data traffic

Faster Loop Detection

• Node can easily detect a loop– Look for its own node identifier in the path– E.g., node 1 sees itself in the path “3, 2, 1”

• Node can simply discard paths with loops– E.g., node 1 simply discards advertisement

32 1

“d: path (2,1)” “d: path (1)”

“d: path (3,2,1)”

Flexible Policies

• Each node can apply local policies– Path selection: Which path to use?– Path export: Which paths to advertise?

• Examples– Node 2 may prefer the path “2, 3, 1” over

“2, 1”– Node 1 may not let node 3 hear the path “1,

2” 2 3

1

04/18/23

Border Gateway Protocol

• Interdomain routing protocol for the Internet – Prefix-based path-vector protocol– Policy-based routing based on AS Paths– Evolved during the past 15 years

• 1989 : BGP-1 [RFC 1105]– Replacement for EGP (1984, RFC 904)

• 1990 : BGP-2 [RFC 1163]

• 1991 : BGP-3 [RFC 1267]

• 1995 : BGP-4 [RFC 1771] – Support for Classless Interdomain Routing (CIDR)

04/18/23

BGP Operations

Establish session on TCP port 179

Exchange all active routes

Exchange incremental updates

AS1

AS2

While connection is ALIVE exchangeroute UPDATE messages

BGP session

Incremental Protocol

• A node learns multiple paths to destination– Stores all of the routes in a routing table– Applies policy to select a single active route– … and may advertise the route to its

neighbors

• Incremental updates– Announcement

• Upon selecting a new active route, add node id to path

• … and (optionally) advertise to each neighbor

– Withdrawal• If the active route is no longer available• … send a withdrawal message to the neighbors

BGP Route

• Destination prefix (e.g,. 128.112.0.0/16)• Route attributes, including

– AS path (e.g., “7018 88”)– Next-hop IP address (e.g., 12.127.0.121)

AS 88Princeton

128.112.0.0/16AS path = 88Next Hop = 192.0.2.1

AS 7018AT&T

AS 12654RIPE NCCRIS project

192.0.2.1

128.112.0.0/16AS path = 7018 88Next Hop = 12.127.0.121

12.127.0.121

04/18/23

ASPATH Attribute

128.112.0.0/16AS Path = 88

AS 1239Sprint

AS 1755Ebone

AS 3549Global Crossing

128.112.0.0/16AS Path = 7018 88

128.112.0.0/16AS Path = 3549 7018 88

AS 88

128.112.0.0/16

Princeton

Prefix Originated

AS 12654RIPE NCCRIS project

AS 1129Global Access

128.112.0.0/16AS Path = 7018 88

128.112.0.0/16AS Path = 1239 7018 88

128.112.0.0/16AS Path = 1129 1755 1239 7018 88

AS7018

AT&T

128.112.0.0/16AS Path = 1755 1239 7018 88

BGP Path Selection

• Simplest case– Shortest AS path– Arbitrary tie break

• BGP not limited to shortest-path routing– Policy-based routing

• Example– Five-hop AS path

preferred over a three-hop AS path

– AS 12654 prefers path through Global Access

AS 3549Global Crossing

128.112.0.0/16AS Path = 3549 7018 88

AS 12654RIPE NCCRIS project

AS 1129Global Access

128.112.0.0/16AS Path = 1129 1755 1239 7018 88

Causes of BGP Routing Changes

• Topology changes– Equipment going up or down– Deployment of new routers or sessions

• BGP session failures– Due to equipment failures, maintenance, etc.– Or, due to congestion on the physical path

• Changes in routing policy– Reconfiguration of preferences– Reconfiguration of route filters

• Persistent protocol oscillation– Conflicts between policies in different ASes

BGP Session Failure

• BGP runs over TCP– BGP only sends updates

when changes occur– TCP doesn’t detect lost

connectivity on its own• Detecting a failure

– Keep-alive: 60 seconds– Hold timer: 180 seconds

• Reacting to a failure– Discard all routes

learned from the neighbor

– Send new updates for any routes that change

AS1

AS2

Routing Change: Before and After

0

1 2

3

0

1 2

3

(1,0) (2,0)

(3,1,0)

(2,0)

(1,2,0)

(3,2,0)

Routing Change: Path Exploration

• AS 1– Delete the route (1,0)– Switch to next route

(1,2,0)– Send route (1,2,0) to

AS 3

• AS 3– Sees (1,2,0) replace

(1,0)– Compares to route

(2,0)– Switches to using AS 2

0

1 2

3

(2,0)

(1,2,0)

(3,2,0)

Routing Change: Path Exploration

• Initial situation– Destination 0 is alive– All ASes use direct path

• When destination dies– All ASes lose direct path– All switch to longer

paths– Eventually withdrawn

• E.g., AS 2– (2,0) (2,1,0) – (2,1,0) (2,3,0) – (2,3,0) (2,1,3,0)– (2,1,3,0) null

1 2

3

0

(1,0)(1,2,0)(1,3,0)

(2,0)(2,1,0)(2,3,0)

(3,0)(3,1,0)(3,2,0)

Time Between Steps in Path Exploration

• Minimum route advertisement interval (MRAI)– Minimum spacing between announcements– For a particular (prefix, peer) pair

• Advantages– Provides a rate limit on BGP updates– Allows grouping of updates within the

interval

• Disadvantages– Adds delay to the convergence process– E.g., 30 seconds for each step

BGP Converges Slowly, if at All

• Path vector avoids count-to-infinity– But, ASes still must explore many alternate paths

– … to find the highest-ranked path that is still available

• Fortunately, in practice– Most popular destinations have very stable BGP routes

– And most instability lies in a few unpopular destinations

• Still, lower BGP convergence delay is a goal– Can be tens of seconds to tens of minutes

– High for important interactive applications

– … or even conventional application, like Web browsing

Interaction with Route Flap Damping

• Motivation for route-flap damping– Flaky equipment goes up and down repeatedly– Leading to excessive BGP update messages– Eventually, want to suppress those updates

• Route Flap Damping– Accumulate a penalty with each routing change– … for each (prefix, peer) pair– Add a fixed penalty for each update message– … and decay the penalty exponentially with time– Apply thresholds to suppress or reuse the route

Operation of Route Flap Damping

Reuse limit

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

0

1000

2000

3000

4000

Time

Penalty

Suppress limit

NetworkAnnounced

NetworkRe-announced

NetworkNot Announced

Interaction with Route Flap Damping

• What are the right thresholds?• Set too high

– Route flaps consume a lot of resources– Users experience more transient disruptions

• Set too low– Regular path exploration triggers suppression– Users experience loss of connectivity

• Not easy to set the parameters “correctly”…– Route-flap damping disabled in many ASes

Research Questions

• How fast does BGP converge?– The n! upper bound is not all that meaningful– What is the tight upper bound?– How does delay depend on the topology?– How does delay depend on the routing policies?

• How to make BGP converge faster?– Any way to skip parts of path exploration process?– Any way to precompute the failover paths?– Any benefits if BGP were a multipath protocol?

• Any way to safely dampen unstable routes?

BGP Modeling: What Problem Does BGP Solve?

• Most do shortest-path routing– Shortest hop count

• Distance vector routing (e.g., RIP)

– Shortest path as sum of link weights• Link-state routing (e.g., OSPF and IS-IS)

• Policy makes BGP is more complicated– An AS might not tell a neighbor about a path

• E.g., Sprint can’t reach UUNET through AT&T

– An AS might prefer one path over a shorter one• E.g., ISP prefers to send traffic through a

customerWhat is a good model for BGP?

Could Use A Simulation Model

• Simulate the message passing– Advertisements and withdrawals– Message format– Timers

• Simulate the routing policy on each session– Filter certain route advertisements– Manipulate the attributes of others

• Simulate the decision process– Each router applying all the steps per prefix

Feasible, but tedious and ill-suited for formal arguments

1

Stable Paths Problem (SPP) Instance

• Node– BGP-speaking router– Node 0 is destination

• Edge– BGP adjacency

• Permitted paths– Set of routes to 0 at

each node – Ranking of the paths

2 5 5 2 1 0

0

2 1 02 0

1 3 01 0

3 0

4 2 04 3 0

3

4

2

1

most preferred…least preferred

5 5 2 1 0

1

A Solution to a Stable Paths Problem

• Solution– Path assignment per

node– Can be the “null” path

• If node u has path uwP– {u,w} is an edge in the

graph– Node w is assigned path

wP

• Each node is assigned – The highest ranked path

consistent with the assignment of its neighbors

2

0

2 1 02 0

1 3 01 0

3 0

4 2 04 3 0

3

4

2

1

A solution need not represent a shortest path tree, or a spanning tree.

An SPP May Have Multiple Solutions

1

0

2

1 2 01 0

2 1 02 0

First solution

1

0

2

1 2 01 0

2 1 02 0

1

0

2

1 2 01 0

2 1 02 0

Second solution

An SPP May Have No Solution

2

0

31

2 1 02 0

1 3 01 0

3 2 03 0

4

3

Ensuring Convergence is Difficult

• Create a global Internet routing registry– Difficult to keep up to date

• Require each AS to publish its routing policies– Difficult to get them to participate

• Check for conflicting policies, and resolve conflicts– Checking is NP-complete– Re-checking for each failure scenario

Research Problems

• Sufficient conditions for global convergence– Restrictions on the topology and routing policies– E.g., based on common types of biz relationships

• Still, an incomplete understanding– Some known combinations of sufficient conditions– Parts of the space are still unexplored

• How do policies affect convergence speed?– Models for the speed of convergence?– Upper bounds on convergence time?

• More on routing policies on Tuesday!